1. Field of the Invention
The present invention relates to a circuit arrangement and controller that generates a high frequency resonant ignition voltage to ignite a high intensity discharge (HID) lamp, to maintain a low frequency square wave lamp current while spikes are superimposed on the low frequency square wave lamp current within an envelope of the low frequency square wave lamp current, to reduce stresses on switching devices during a polarity transition of the lamp voltage or lamp current when the HID is being extinguished, and to improve a lamp glow-to-arc transition by applying an imbalanced high frequency current after a lamp breakdown.
2. Discussion of Background and Relevant Information
Two distinctly different methods exist to ignite an electronic high intensity discharge (HID) lamp. In a first method, the lamp is ignited using a pulsed method. In a second method, the lamp is ignited using a resonant method. Because of safety concerns associated with a peak magnitude of an ignition voltage, the resonant method, which requires a voltage having a smaller peak magnitude (in comparison with the pulsed method), remains the preferred technique for use in electronic high intensity discharge (HID) lamp ballasts.
Two distinctively different methods of operating the lamp after ignition are employed with electronic high intensity discharge lamp ballasts. In a first method, the lamp is operated at a high frequency range of, for example, several tens of kilohertz. In a second method, the lamp is operated at a low frequency range of, for example, several hundreds of hertz. Because of acoustic resonance problems associated with operating the lamp in the high frequency range, it is preferable to operate the lamp at the low frequency range.
FIGS. 8 and 9 illustrate two approaches for generating a high frequency voltage with sufficient energy to ignite the lamp and to run a lamp in a low frequency operation. In FIG. 8, a discharge lamp driving circuit includes a combined chopper and high frequency inverter. Depending upon the control scheme implemented with switches Q1 to Q4, this configuration can serve many design purposes.
U.S. Pat. No. 4,912,374 describes one such implementation. It is well known that HID lamps may exhibit an acoustic resonance when it is operated at a high frequency. U.S. Pat. No. 4,912,374 discloses a method that reduces the acoustic resonance problem by interrupting the high frequency current with a smoothed DC current. Buck inductor L1 and buck filter capacitor C1 form a buck resonant network. Transformer T and ignition capacitor C2 form an inverting resonant network. If semiconductor switches, such as, for example, transistor switching pair Q1 and Q4 and transistor switching pair Q2 and Q3 are complementarily switched at a high frequency rate, two high frequency AC currents will flow through the lamp. A first high frequency AC current will flow through the capacitor C1 and the inductor L1. A second high frequency AC current will flow through the ignition capacitor C2 and the transformer T.
As a result, a loop current is formed between the buck filter capacitor C1, the transformer T, and the lamp. In a chopper (or buck) configuration, switch Q1 is ON and switches Q2 and Q3 are completely OFF while switch Q4 is switched at a high frequency rate. As a result, a DC current flows through the lamp from left to right (of the circuit) during this period. When switches Q1 and Q2 change state (e.g., the operation state of switch Q1 changes from ON to OFF and the operation state of switch Q2 changes from OFF to ON), the voltage at the junction of switches Q1 and Q2 changes from HIGH to LOW within a very brief period of time, such as, for example, a couple of hundred nanoseconds. This rapid (sudden) voltage change causes a spike current to flow out of the ignition capacitor C2 and the lamp, from the left to the right, and for current to flow out of the buck filter capacitor from right to left, back to the junction of switches Q1 and Q2.
Note that the direction of the spike current is the same as the direction of a DC lamp current. When switch Q3 is switched at a high frequency rate, switch Q2 is ON and switches Q1 and Q4 are completely OFF. Thus, a DC current flows from right to left through the lamp during this period. When switch Q2 changes from an ON state to an OFF state and switch Q1 changes from an OFF state to an ON state, the voltage at the junction of switches Q1 and Q2 changes from LOW to HIGH within a couple of hundred nanoseconds. This rapid (sudden) change on voltage causes a spike current to flow into the ignition capacitor C2 and the lamp from right to left, and for current to flow in the buck filter capacitor C1, from left to right. The spike current has the same direction as the DC lamp current. Unfortunately, in both cases, the spike current re-enforces the DC lamp current, causing the instantaneous lamp current at the spike to be higher than the average DC current. In the case where only a DC current (e.g., low frequency square wave current) is applied to the lamp during a normal operation, the spike current is higher than an envelope of the low frequency square wave current. This is not desirable.
A more detailed explanation on how the circuit behaves during the polarity transition of the lamp voltage or lamp current will now be provided. Generally, during the transition, the four switches Q1 to Q4 operate as shown in FIG. 10. During a first half cycle, switch pair Q1 and Q4 is active. At the end of the first half cycle, all switches are turned OFF at time t equals t1 to avoid cross conduction. Then, a so-called xe2x80x9cdead timexe2x80x9d begins, during which time there is no electrical conduction. After the dead time (e.g., when time t equal t2), switching pair Q2 and Q3 become active. The load current reverses its polarity and flows in the opposite direction, as compared with the active half cycle of switch pair Q1 and Q4. Because of the switching at time t equals t1, the voltage at the junction of switching pair Q1 and Q2 suddenly goes from being substantially equal to bus voltage V(1), to either float or become substantially equal to a negative rail voltage, to continue xe2x80x9cfree-wheelingxe2x80x9d. Unfortunately, this instantaneous change in voltage causes a spike current to flow through the ignition capacitor C2 and the lamp in the same direction as the low frequency square wave current.
FIG. 9 illustrates a modification of U.S. Pat. No. 4,912,374. According to this circuit, switch Q5 and diode D5 are added, so that the lamp current exhibits a clean square wave. Switch (e.g., MOSFET) Q5 is turned (switched) OFF after the lamp is ignited, or whenever the high frequency current is not needed for the operation of the lamp. When switch Q5 is switched OFF, the ignition capacitor C2, is electrically disconnected from the circuit. Thus, no current flows through the ignition capacitor C2 and the lamp, due to the switching of switch pair Q1 and Q2. Diode D5 functions to prevent any voltage overshoot during the switching of switch (MOSFET) Q5.
Unfortunately, modifying the circuit of U.S. Pat. No. 4,912,374 to include the high voltage MOSFET Q5, the high voltage diode D5, and a driving circuitry required to drive switch Q5 increases the complexity of the lamp driving circuit. Further, the inclusion of these additional components increases manufacturing costs.
The lamp voltage (or lamp current) may be sensed to detect a light dropout during a normal operation. If the lamp voltage exceeds a predetermined maximum voltage for the lamp to operate normally, the lamp is determined to have dropped out (e.g., light from the lamp is extinguished). The controller is then quickly switched from a (normal) operating mode to a starting (ignition) mode to re-ignite the lamp. The transition from the operation mode to the starting mode usually requires at least one low frequency cycle. Any time duration less than approximately one low frequency cycle could result in a false lamp dropout detection.
An analysis of the circuit operation will now be provided, with reference to FIGS. 8 and 11. Assume that the lamp is extinguished when the switch pair Q2 and Q3 is active after the duty cycle is determined. The lamp voltage stays the same for the switch pair Q2 and Q3 pair until the end of the half cycle. During the next half cycle, switching pair Q1 and Q4 become active. Since the lamp has dropped out (e.g., been extinguished) and there is no current flowing, switching pair Q1 and Q4 stays ON for the entire half cycle. As a result, the buck filter capacitor C1 is fully charged to become substantially equal to the bus voltage V(1).
At the completion of this half cycle, another half cycle starts, in which switching pair Q2 and Q3 is turned ON. The voltage across the buck inductor L1 is substantially equal to twice the bus voltage V1 because of the voltage on the buck filter capacitor C1. The current in the buck inductor L1 rises linearly in a very short period of time to a predetermined limit. As a result, switch Q3 turns OFF while switch Q2 remains ON. The current in the buck inductor L1 freewheels through an internal diode (not shown) of switch Q4, causing the entire buck filter voltage to be dropped across the buck inductor L1. The polarity of this voltage is the same as when switch Q3 is ON. Thus, the current in the buck inductor L1 continues to rise while the voltage on the buck filter capacitor C1 slowly drops (decreases). Eventually, the buck inductor L1 saturates, causing a large freewheeling current. It should be noted that switches Q1 through Q4 should be selected so as to be strong enough to handle the current without any characteristic degradation.
The present invention overcomes the drawbacks discussed above, in which the spike lamp current superimposed on the low frequency square wave lamp current is higher than an envelope of the low frequency square wave lamp current. The present invention reduces the high current stress that would otherwise be imposed on the semiconductor switches during the lamp current (or voltage) polarity transition, especially when the lamp has just been extinguished, and the insufficient electrode heating during starting after lamp breakdown by a high frequency current.
Accordingly, an object of the present invention is to provide a smooth low frequency square wave lamp current during a normal operation mode, even if a spike current is superimposed on the low frequency square wave lamp current. Any spike current superimposed on the low frequency square wave lamp current will be within an envelope of the low frequency square wave lamp current, so that the spike current does not change the characteristics of the low frequency square wave lamp current.
Another object of the present invention is to reduce current stresses imposed on semiconductor switches during a polarity transition (change) of the lamp voltage (or lamp current) after the lamp extinguishes. According to the instant invention, when the lamp extinguishes, part of the energy stored in the buck filter capacitor C1 is released to the DC bus line, reducing electrical stresses on the various electrical components.
Another object of the present invention is that, during a starting (ignition) mode after the lamp breaks down, an imbalanced high frequency current is supplied to the lamp. The net imbalance of the high frequency lamp current is a DC current that increases the RMS current of the lamp that helps the lamp pass a glow-to-arc transition.
According to an object of the present invention, an inverter circuit that drives a lamp includes a buck filter network; an ignition network including at least an ignition capacitive device; a plurality of switching devices; and a controller. The controller controls an ON time and an OFF time of each switching device, so as to effect a polarity transition of at least one of a lamp voltage and/or a lamp current. The controller also inserts a predetermined dead time delay before the polarity transition.
According to an advantage of the invention, the predetermined dead time delay may be variable, and, for example, may be varied in accordance with a generated load voltage.
According to another advantage of the invention, a lookup table is used to store data related to a length of time of the predetermined dead time delay. The predetermined dead time delay operates to minimize a lamp current spike during the polarity transition by ensuring that the spike superimposed on the lamp current is smaller than an envelope of a square wave low frequency lamp current.
According to another object of the present invention, an inverter circuit that drives a lamp is disclosed having an ignition network that includes at least an ignition capacitive device; a plurality of switch devices; and a controller that controls a duty cycle of each switch device of the plurality of switching devices. The duty cycle creates a polarity transition in one of a lamp voltage and a lamp current. A dead time delay of a predetermined time (which may be variable) is established before the polarity transition. It is noted that the plurality of switching devices may be a full bridge.
According to an advantage of the invention, the predetermined time is varied in accordance with a load voltage generated by the driving circuit. In this regard, it is noted that data related to the predetermined time may be stored in a lookup table.
According to an advantage of the present invention, the dead time delay operates to minimize a lamp current spike during the polarity transition by ensuring that the spike superimposed on the lamp current is smaller than an envelope of a square wave low frequency lamp current.
A still further object of the present invention pertains to an inverter circuit that drives a lamp that includes a buck filter network with a capacitance device, such as, for example, a capacitor; and at least one switch pair that has a predetermined duty cycle and which is turned ON and OFF for several high frequency cycles at a beginning of a low frequency half cycle.
According to an advantage of the invention, the switch pair is turned ON and OFF in accordance with a load voltage applied to the driving circuit. In particular, the predetermined duty cycle may be set in accordance with a sensed load voltage generated by the driving circuit. Further, the predetermined duty cycle may be varied in accordance with the sensed load voltage.
An advantage of the invention resides in that at least one switch pair is turned ON and OFF for several high frequency cycles at a beginning of a low frequency half cycle to discharge an energy stored in a buck filter energy storage device of the buck filter network. In particular, the energy stored in the buck filter energy storage device may be discharged to a DC voltage source, such that a voltage on the buck filter energy storage device is approximately equal to zero at a start of a half cycle.
Another object of the present invention pertains to a method for driving a lamp, comprising providing an ignition network to ignite a lamp; providing a buck filter network to continue driving the lamp after the lamp is ignited; and controlling a duty cycle of at least one switching pair to effect a polarity transition of at least one of a lamp voltage and a lamp current to insert a predetermined dead time delay before the polarity transition.
According to an advantage of the method, a duty cycle of at least one switching pair is controlled to insert a variable dead time delay. The dead time delay may be varied in accordance with a load voltage. Further, data related to the dead time delay may be stored in a lookup table. Controlling a duty cycle operates to minimize a lamp current spike during the polarity transition by ensuring that the spike superimposed on the lamp current is smaller than an envelope of a square wave low frequency lamp current.
A still further object of the invention pertains to a method for driving a lamp, comprising using an ignition network to ignite a lamp; using a buck filter network to drive the lamp after the lamp is ignited; and controlling at least one switch pair to have a predetermined duty cycle, wherein the at least one switch pair is turned ON and OFF for several high frequency cycles at a beginning of a low frequency half cycle.
The method of the present invention operates to control at least one switch pair to turn ON and OFF in accordance with a load voltage. The predetermined duty cycle may be set in accordance with a sensed load voltage. Accordingly, the predetermined duty cycle may be varied in accordance with the sensed load voltage.
According to an advantage of the present method, the at least one switch pair is turned ON and OFF for several high frequency cycles at a beginning of a low frequency half cycle to discharge an energy stored in a buck filter energy storage device of the buck filter network. Further, the energy stored in the buck filter energy storage device may be discharged to a DC voltage source, such that a voltage on the buck filter energy storage device is approximately equal to zero at a start of a half cycle.